Re: how to use expand_builtin_alloca

[fche at redhat dot com (Frank Ch. Eigler)]

> Janboe Ye <yuan-bo.ye@motorola.com> writes:

>> normally gcc will use expand_builtin_alloca to handle variable array.
>> But mudflap will force this function to return immediately to invoke
>> alloca explicit.
>>
>> Is there some way to still use expand_builtin_alloca without changing
>> gcc source code?

I don't think so.


Ian Lance Taylor <iant@google.com> writes:

> mudflap can't check accesses to memory allocated using alloca unless
> it overrides __builtin_alloca.

It can't currently.  But instead of redirecting the call to a
heap-based alloca() wannabe in libmudflap/mf-hooks1.c, perhaps
mudflap could instrument alloca() by generating code like this
instead:

  __builtin_alloca(N)  -->  GIMPLE_TRY_FINALLY( try {
                                ptr = __builtin_alloca(N)
                                __mf_register(ptr ...)
                                ptr;
                           } finally (attached to the function scope) {
                                __mf_unregister(ptr ...)
                           }

Or perhaps not, if alloca() can be used in loops in way that
prevents clean nesting of the try/finally.  

OTOH, I believe the original poster's case came from gcc-synthesized
alloca's, coming from variable-length array allocation.  Those in turn
might be represented with almost the normal mf_xform_decls(), while
letting __builtin_alloca() remain.

Either of these requires gcc changes though.


> [...]  Although, of course, you could simply not use mudflap for the
> code in question.

The original poster's purpose is specifically to build bits of the
linux kernel with mudflap instrumentation.


- FChE